(1) Field of the Invention
The invention relates to the photographic process known as vesicular imaging, and to the imaging element used therein. In particular, it concerns novel binders which contain, in layer form, the radiation-sensitive vesiculating agent which imagewise decomposes to form microscopic lightscattering vesicles of gas in the binder. Vesicular films are of considerable importance in information storage and retrieval, such as by microfilming, due to the facts that vesicular images have very high resolution, and are stable in ambient light and normal use temperatures. Further, since vesicular film has a low infrared absorption, it is superior to silver transparencies, as noted in Kosar, Light Sensitive Systems, p. 280 (1965). Particularly such is the case when the original image is reduced, as in microfilming, by a factor greater than 25 times.
(2) State of the Prior Art
The first commercial vesicular photographic materials utilized gelatin binders. Gelatin was replaced with improved binder materials due to the undesired tendency of gelatin to absorb moisture and therefore to release the image-forming bubbles, destroying the image. Typical improved materials included a great variety of synthetic polymers, such as poly(vinyl chloride), poly(vinylidene chloride), and polystyrene; and copolymers obtained by copolymerizing acrylonitrile with vinyl chloride, styrene, vinylidene chlorofluoride, or 1,1-difluoroethylene; by copolymerizing vinyl chloride with methyl acrylate, acrylic acid, diethyl maleate, or vinyl acetate; or by copolymerizing vinylidene chloride with vinyl chloride, vinyl acetate, vinyl alcohol, ethyl acrylate, or acrylonitrile. Examples of the homo- or co-polymerization of vinylidene chloride are described in U.S. Pat. No. 3,032,414, issued to R. James.
To be suitable, a binder must be sparingly permeable, must have the proper diffusivity for the light-generated gas, and must have the proper rigidity for generating the imageforming vesicles. Of these 3 characteristics, permeability and diffusivity are approximately proportionate, and even rigidity is related to permeability, as is noted in the aforesaid U.S. Pat. No. 3,032,414. Thus, it is generally sufficient to evaluate only the permeability constant (PC) of a material to determine if the material is satisfactory as a binder. This constant is most readily expressed as PC, equal to the number of cubic centimeters of gas transmitted by 1 sq. centimeter of the binder material during one second at constant temperature, when the pressure gradient is one centimeter of mercury per centimeter of the thickness of the binder layer, or cm.sup.3. cm.sup..sup.-2. sec.sup..sup.-1. (cm. Hg/cm).sup..sup.-1. It has been established that the binder should have a PC of between about 1.times. 10.sup..sup.-11 as PC.sub.max, and about 1.times. 10.sup..sup.-15 as PC.sub.min, at a temperature of 30.degree. C. See, for example, U.S. Pat. No. 3,355,295 to Priest. These limits appear to be useful regardless of the gas involved in the vesicular image formation.
The previously-described binder comprising a copolymer of vinylidene chloride and acrylonitrile, as particularly set forth in said U.S. Pat. No. 3,032,414, meets the above-stated range for PC. However, at normal temperatures the copolymer tends to release hydrogen chloride, which corrodes the metal commonly used in storage containers for the photographic element made therefrom.
The undesirability of vinylidene chloride has been solved in part by the use of .alpha.-chloroacrylonitrile as the predominating (by weight) comonomer, examples of which are taught in U.S. Pat. Nos. 3,620,743, 3,622,335 and 3,622,336 issued to Notley. However, vesicular imaging materials using these binders have exhibited lower sensitivity than similar materials using binders of the invention. (See Examples 16-18 below.)
Still other examples of binders include polyamides, such as those disclosed in U.S. Pat. No. 3,383,213 issued to Notley et al. However, as will be shown in the Examples below, these binders do not give reproducible vesicular imaging results, possibly because of improper permeability.
Other patents pertaining principally to the general background and development of the field of vesicular imaging elements include U.S. Pat. Nos. 3,260,599 issued July 12, 1966, 3,208,850 issued Sept. 28, 1965, 2,923,703 issued Feb. 2, 1960, 2,703,756 issued Mar. 8, 1955, and 2,699,392 issued Jan. 11, 1955.
Patent disclosures already published and pertaining to sulfonamide polymers used, however, other than as vesiculating binders, include British Patent Specification No. 1,353,542 and German Offenlegungsschrift No. 2,352,348.